Lyotropic liquid crystalline polyamides, such as poly(p-phenylene terephthalamide), which are exemplified by such commercially available products as TWARON polyamide and KEVLAR polyamide, exhibit liquid crystalline properties in solution.
Polymers which demonstrate liquid crystalline properties in the melt, and which are referred to as thermotropic liquid crystalline polymers, have been limited chiefly to poly(esters) such as VECTRA brand polyester. In general, aromatic polyamides have not demonstrated thermotropic behavior due to their extremely high melting points as a result of intermolecular hydrogen bonding and the rigidity of the aromatic mesogen which is necessary to convey liquid crystalline properties. Consequently decomposition occurs before melting. There have been a few examples of amide-containing thermotropic liquid crystalline polymers in the literature which have either large flexible spacers or have large substituents to lower the melting temperature of the polymer. Some examples of such polymers include:
1. J. M. G. Cowie et al., in British Polymer Journal, 20, 515-519 (1988) shows thermotropic liquid crystalline main chain polyamides containing diaza-18-crown-6-ether units.
2. M. Schmucki et al., in Makromol. Chem. 190, 1303-1308 (1989) show polyamides with stiff and flexible chain segments which are formed by reaction of a dicarbonyl dichloride, optionally containing a short alkylene bridge group between the phenyl rings with an ortho-substituted .alpha.,.omega.-bis(4-aminophenyl) alkylene monomer.
3. A. C. Griffin et al., in Mol. Cryst. Liq. Cryst. Vol. 82 (Letters), pp. 145-150, shows thermotropic polyamide liquid crystalline materials formed from the polymerization of 4,4'-dichloroformyl-1,10-diphenoxydecane and a 3,3'-disubstituted-4,4'-diaminobiphenyl monomer.
4. Uryu et al., polymer Journal, Vol. 21, No. 12, pp. 977-986 (1989), describes thermotropic liquid crystalline copoly(ester-amides) which are formed from 4,4'-diacetoamido-3,3'-dimethoxybiphenyl or 4,4'-diacetoamido-3,3'-dichlorobiphenyl and diacetylated p-phenylenediamine, in combination.
5. H. Ringsdorf et al., in Makromol. Chem. 188, 1431-1445 (1987), illustrate liquid crystalline rigid-rod polyesters and polyamides containing disk-like mesogens in the main chain which are derived from discoid 1,4-hydroquinone derivatives.
U.S. Pat. No. 5,070,155 to M. Liu et al. describes semi-aromatic copolyamide or copolyesteramides which are prepared from an aliphatic polyamide and an aromatic hydroxyacid or aminoacid.
D. Liu et al. in Polymer Preprints, Vol. 33, No. 2 (August 1992), indicate that thermotropic aromatic-aliphatic polyesteramides can be formed based on p-terephthaloyl chloride, dimethylbenzidine, and hexylene glycol.